1. Field of the Invention
The present invention relates to a print head adaptable to an optical printer, and more particularly to a print head having a field emission device.
2. Related Art
Hitherto, optical printers have been known. The schematic structure of the optical printer will now be described with reference to FIG. 1. A film 120 is coated with a sensitive material, such as silver halide (silver salt), so as to be exposed to light when the lower surface of the film 120 is irradiated with light reflected by a mirror 121.
The film 120 is irradiated with light emitted from a print head 125. The print head 125 is supplied with image data for each line. Light modulated by image data above is main-scanned vertically on the surface of the sheet and the print head 125 is sub-scanned as indicated by an arrow shown in FIG. 1 so that one image is printed on the film 120 by a line sequential method.
Reference numeral SLA 122 represents a SELFOC lens array serving as a lens for causing light emitted from the print head 125 to be focused on the surface of the film 120. A mirror 123 introduces light into the SLA 122.
An RGB filter 124 is an optical filter of three primary colors for printing a color image on the film 120. In a case where a color image is printed, image data for one line is decomposed into R (red), G (green) and B (blue) image data, and then the RGB filter 124 is sequentially moved to correspond to image data for each color so that the RGB filter 124 performs the main scanning operations three times. That is, the main scanning operations performed by three times result in the color image for one line being recorded on the film 120.
An optical printer of the foregoing type has a light source which has been a light emitting diode (LED) or a fluorescent character display tube of a thermionic emission type. In recent years, use of semiconductor microprocessing technique has enabled micron size field emission devices to be formed into an array configuration on a substrate. A field emission print head using the foregoing field emission device array as the electron source has been suggested (refer to Japanese Patent Laid-Open No. 4-43539).
An example of the structure of a conventional field emission print head of the foregoing type is shown in FIGS. 2A, 2B and 2C. FIG. 2A is a schematic plan view, FIG. 2B is a schematic cross sectional view taken along line 2B--2B shown in FIG. 2A, and FIG. 2C is a detailed cross sectional view taken along line 2C--2C shown in FIG. 2A. As shown in FIGS. 2A, 2B and 2C the field emission print head has a first flat substrate 101 having a plurality of field emission devices 105 formed thereon, a second flat substrate 102 disposed opposite to the first flat substrate 101 and having a fluorescent member 106 and so forth formed thereon, a holder member 103 for maintaining a predetermined distance from the first flat substrate 101 to the second flat substrate 102, and a vacuum layer 104 surrounded by the first flat substrate 101, the second flat substrate 102 and the holder member 103.
The first flat substrate 101 is made of an n-type silicon single crystal substrate and covered with a silicon oxide film (SiO.sub.2 film) 101' except the field emission devices 105 and the substrate contact electrode 107 thereof. The second flat substrate 102 is made of a transparent glass substrate and having a transparent anode electrode 109 and a fluorescent member 106 laminated on the surface thereof. The field emission devices 105, each having a cathode electrode and a gate electrode, and the fluorescent member 106, having an anode electrode, are disposed to opposite to each other in such a manner that a vacuum layer 104 is formed between the field emission devices 105 and the fluorescent member 106. A pair of the field emission device 105 and the fluorescent member 106 form a unit light source. Each unit light source has one field emission device sectioned by gate electrodes separated from one another and disposed in the form of an array. The cathode electrode of each of the field emission device shares a monocrystal silicon plate. Also the anode electrode is commonly shared.
One field emission device, as shown in FIG. 2C, has a plurality of projecting cathode electrodes (emitters) 111 formed on the surface of the first flat substrate 101 and gate electrodes 112 formed on the SiO.sub.2 film 101' and having openings adjacent to the foregoing projections. The gate electrodes 112 are separated from one another by each field emission device.
Although the first flat substrate 101 is made of the single crystal silicon substrate and the projections are formed by anisotropic etching of the single crystal silicon substrate, an insulating substrate having metal electrodes and metal projections may be employed or a structure having metal projections formed on a conductive substrate may be employed.
In the thus-structured unit light source in a state where the single crystal silicon substrate 101 is grounded through the substrate contact electrode 107, when anode voltage V.sub.ak is applied to the fluorescent member 106 through the anode contact electrode 110 and the anode electrode 109 and gate voltage V.sub.gk is applied to the gate electrode of the field emission devices 105 through the gate contact electrode 108, the electric field of the gate electrode is applied to the projection portions of the cathode electrode of the field emission devices 105 so that electrons are field-emitted from the leading portions of the projections. The field-emitted electrons are accelerated due to the anode voltage when allowed to reach the fluorescent member 106 so that the portions of the fluorescent member 106 opposite to the device emit light.
Thus-emitted light passes through the transparent anode electrode 109 and the second flat substrate 102 to be radiated so that image data for one line is emission-recorded on a recording medium, such as a film. In the foregoing case, the line sequential scan method may be employed as described above, in which the recording medium or the print head is moved to record image data for the following one line. When the RGB filter 124 is, as shown in FIG. 1, moved to perform main scanning, a color image can be recorded.
Since a field emission print head of the foregoing type is manufactured by using the microprocessing technique for semiconductors, high resolutions can be realized.
However, in the foregoing conventional field emission print head, electrons are emitted from the leading ends of the projecting cathode electrodes 111 for field-emitting electrons while being spread by an angular degree of about 60 degrees.
Therefore, somewhat spread electrons reach the anode. As a result, there is a risk that electrons collide with adjacent pixels in the anode portion, and thus there arises a problem in that leakage light emission takes place.
In order to prevent the foregoing problem, the pixel pitch is required to be elongated. However, elongation of the pixel pitch causes the resolution to deteriorate. Although the spread of electrons can be prevented by shortening the distance from the cathode to the anode, the distance cannot be shortened because the apparatus cannot withstand the operation voltage of hundreds of volts which is applied to the anode in the case where the distance from the cathode to the anode is shortened.